Sunday, 31 March 2013

Let's talk Chrome-Moly - Part 2

Part 2 - Martyr DOM

In Part 1 we talked about the strength differences between 1020 DOM and Chrome-Moly, two materials commonly used in the construction of roll cages. We concluded that the differences were not sufficient enough to justify the price alone, but Chrome-Moly still has a chance here.

In this part, we're going to go through some fabrication differences as well as comments from people who have worked with both materials.

Only as strong as your weakest link.

Steel sheets are formed and rolled in highly consistent, standardized conditions. Your welds are not. Generally speaking, the quality of the weld will determine the strength of the frame. Tubes (which are what most racing roll cages are made out of) are great at handling axial forces, but those axial forces must be transferred and redirected at welded joints. You have a ton of forces coming from different directions at a joints, as well as stress from rapid expansion and contraction during welding at joints.

How easy are 4130 and 1020 to weld exactly?

Chrome-Moly isn't easy to weld. Sources vary on how easy it can be, suggesting everything from being able to weld it straight (due to purity) to requiring preparation. Erring on the safe side, I'll outline some hiccups to welding it:

Chrome-Moly doesn't take thermal shock well. That means stresses and cracks from thermal expansion and contraction are more prevalent than other lower-carbon steels (like 1020 DOM, which has 0.10% lower Carbon per unit wt)

This means it has to be heated slowly. And not only that, the frame around the weld must be preheated so it doesn't suck heat too fast from the weld.

It also has to be cooled very slowly. Air cooling is too fast, and it is preferred to play with the weld using a torch to cool it to red.

From the same source, Chrome-Moly is generally TIG welded because MIG welding injects heat too fast.

Additional Comments

We are by far not the first team to have a decision like this. Here are some other comments from welding forums, buggy enthusiasts and small aircraft enthusiasts:

"After lengthy research I concluded that to make a worthwhile 4130
chromoly frame I would need to pre-treat my materials, TIG weld
everything, then properly stress-relieve the weld areas (oven heat
treatment, normalizing, etc). Moreover if I or my customers wanted to
modify, add-to, or repair a frame, I/they would have to have access to
all of this equipment... Most fabricators agree 4130 chromoly is not much stronger unless appropriately treated... 1020 DOM is easy to work with: you the customer can modify and/or
repair the frame with a MIG welder. No special equipment or processes
needed."

"1020 DOM is usually safer for a roll cage because of its greater elongation properties. The
1020 will continue to bend after the 4130 has ruptured. Sometimes 4130 is used
because it is processed in a more controlled enjoinment and certified for
aircraft use compared to commercial grade 1020... If you don't heat treat for
greater strength, then just use 1020 DOM to reduce costs, not just in material
but tools wear out faster with 4130."

Conclusions

Despite all the flaws of Chrome-Moly, why do people use it? That's a good question, and based on my brief research, the conclusion seems to be that 4130 Chrome-Moly steel is:

able to achieve higher strength than 1020 DOM with heat treatment

it does require work, however if strength to weight is more critical than factors such as cost, time and ease of fabrication, then nobody will question your priorities

available in a purer form than 1020 DOM (created in more tightly controlled conditions)

Even with all my marks against it, I cannot deny that in the right hands (such as those constructing airplanes) Chrome-Moly steel is a better choice for highly weight-critical applications. But those choosing it just to say they have exotic aircraft steel in their project should beware: cheaper solutions can work just as well and be less of a pain. 1020 DOM is a better choice for strength-to-weight vs. cost vs. ease of construction.